How to Update Array dict Elements in mongodb based on another field - arrays

Question

How can I update a value in a document based on applying functions to another field (which is in a different embedded document)?
With the sample data below, I want to
get the col field for the farm having id 12
multiply that by 0.025
add the current value of the statistic.crypt field
ensure the value is a double by converting it with $toDouble
store the result back into statistic.crypt
data:
{
"_id": {
"$oid": "6128c238c144326c57444227"
},
"statistic": {
"balance": 112570,
"diamond": 14,
"exp": 862.5,
"lvl": 76,
"mn_exp": 2.5,
"lvl_mn_exp": 15,
"coll_ms": 8047,
"all_exp": 67057.8,
"rating": 0,
"crypt": 0
},
"inventory": {
"farm": [{
"id": 12,
"col": 100,
"currency": "diamond",
"cost": 2,
"date": "2021-09-02 18:58:39"
}, {
"id": 14,
"col": 1,
"currency": "diamond",
"cost": 2,
"date": "2021-09-02 16:57:08"
}],
"items": []
},
...
}
My initial attempt is:
self.collection
.update_many({"inventory.farm.id": 12}, [{
"$set": {
"test": {
'$toDouble': {
"$sum": [
{'$multiply':["$inventory.farm.$[].col", 0.025]},
'$test'
]
}
} }
},])
This does not work as it applies to test rather than statistic.crypt, and I cannot figure out how to modify it to apply to statistic.crypt.

Answer 1

A field can be updated based on another in the following stages:
add a field containing the farm
set statistic.crypt to the result of the mathematical expression (applied to the newly embedded farm)
remove extra fields
In code:
self.collection.update_many({"inventory.farm.id": 12 }, [
{
$addFields: {
hh: {
$filter: {
input: "$inventory.farm",
as: "z",
cond: { $eq: ["$$z.id", 12] },
},
},
},
},
{
$set: {
"statistic.crypt": {
$toDouble: {
$sum: [
{
$multiply: [{ $first: "$hh.col" }, 0.025],
},
"statistic.crypt",
],
},
},
},
},
{
$project: {
id_pr: 1,
id_server: 1,
role: 1,
warns: 1,
id_clan: 1,
statistic: 1,
design: 1,
date: 1,
inventory: 1,
voice: 1,
},
},)